Particle formation using supercritical fluids has been researched for many decades, and has resulted in the development of numerous processes using the supercritical fluid (SCF) either as solvent, such as in the “rapid expansion of supercritical solutions” (RESS) process, as antisolvent in the “gas antisolvent” (GAS) process, or as co-solvent in the “depressurization of an expanded liquid organic solution” (DELOS) process. Numerous variations and further developments of particle formation processes have emerged [1-4]. However, many of these processes require organic solvents to dissolve the solute to be precipitated (US Patent application publication Ser. No. US 2004/0110871 A1 to Perrut et al.).
In International Publication Nos. WO 1995/01221 and WO 1996/00610, methods are described for the formation of particles based on the antisolvent method, in which an aqueous solution of a substance is contacted with a co-solvent (also referred to as modifier) (such as ethanol) and a supercritical fluid in a coaxial nozzle. As stated in WO 1996/00610, the supercritical fluid may optionally contain a low level of a co-solvent (i.e. ethanol), preferably not more than 20%. However, such a low level of co-solvent is not sufficient to precipitate high molecular weight biopolymers such as polysaccharides.
Another approach to form β-carotene nanoparticles was presented by Cocero et al. [5], which is based on the ‘supercritical fluid extraction of emulsions’ (SFEE) process, related to the GAS process. In the SFEE process, a nanoemulsion of an organic solvent (dichloromethane) carrying the solute is dispersed in water to form an oil-in-water emulsion and dried using supercritical CO2 (SC—CO2). Each droplet resembles a small GAS precipitator, where upon expansion with CO2 and extraction of the organic solvent, ultrasmall particles suspended in water are formed with a final organic solvent concentration of about 1 ppm [5].
In the DELOS process [6], the solute is first dissolved in an organic solvent and a compressed fluid such as CO2 is added to expand the solution at the desired temperature and pressure. Then, the expanded solution is rapidly depressurized to atmospheric conditions, resulting in the formation of sub-micron or micron sized solute crystals due to the very large temperature drop that occurs upon depressurization.
Particle formation processes are known for the treatment of aqueous solutions containing the solute, which are sprayed into a high pressure precipitation chamber together with pressurized CO2 enriched with ethanol. This approach is often referred to as the supercritical fluid drying process, which has been applied to precipitate proteins [7], enzymes [8], lactose, maltose, trehalose, raffinose, cyclodextrin, low-molecular-weight dextrans, mid-molecular-weight dextrans up to about 68,800 g/mol, and inulin [9], forming free-flowing powders. The effect of spraying conditions and nozzle design as well as the influence of various co-solvents added to CO2 on the shape and size distribution of particles obtained with supercritical fluid drying has been studied by Bouchard et al. [10, 11]. It was found that methanol and ethanol used as a co-solvent in the SCF drying process acted as antisolvent in the precipitation of glycine, phenylalanine and lysozyme, besides their role in enhancement of water solubility in SC—CO2 and evaporative water removal, whereas 2-propanol and acetone did not act as an antisolvent and affected mainly the evaporative water removal [11]. Various nozzle configurations were tested as well in the SCF drying process, including a simple T-mixer with small inner diameter or coaxial converging nozzles, with and without mixing chamber, as well as with ultrasonic wave generator [10], which showed that the nozzle design, processing pressure and flow rates had a pronounced effect on particle size, whereas morphology was found to be more likely linked to the precipitation mechanism rather than the atomization process [10]. A mathematical model for the mass transfer from an aqueous drop to SC—CO2+ethanol was developed to study the drying of aqueous solutions of lysozyme with CO2+ethanol mixtures [12].
Another approach was applied using SC—CO2 for drying of aqueous green tea extracts, employing a variation of the ‘particles from gas-saturated solutions’ (PGSS) process, using only pressurized CO2 as the drying medium in a spray chamber at a mild temperature ranging from 30 to 60° C. and 20 MPa to obtain free-flowing powders, containing the intact active ingredients, such as antioxidant polyphenols. Kluge et al. [13, 14] applied the SFEE process to obtain composite nanoparticles of an anti-inflammatory drug (Ketoprofen™) and amorphous biodegradable polymer poly-lactic-co-glycolic acid (PLGA), finding that the PLGA concentration in the emulsion affected particle size and particle size distribution. A process was described for the encapsulation of lavandin essential oil in a matrix of n-octenyl succinic anhydride (OSAN)-modified starch by spraying an aqueous emulsion of the oil with SC—CO2 applying a PGSS drying technique, where the emulsion was continuously mixed with CO2 at 10 MPa and sprayed into a precipitation chamber at atmospheric pressure [15]. The oil was also encapsulated in polyethylene glycol (PEG) applying a PGSS technique, where the PEG was used in a molten form containing pressurized CO2 forming a gas-saturated solution, which was mixed with the lavandin oil and sprayed into a precipitation chamber at atmospheric pressure [15].
The prior art does not provide any solutions regarding the formation of micro/nano-sized particles, agglomerates or fibers (micro- or nanoparticles) from water-soluble high molecular weight (HMW) biopolymers, such as HMW gums and polysaccharides with molecular weights ranging from about 70,000 g/mol (70 kDa) up to over 1,000,000 g/mol (1,000 kDa), applying a SCF drying (SFD) and/or gas antisolvent (GAS) technique. As known to those skilled in the art HMW biopolymers, in particular polysaccharides, form highly viscous solutions. This is a major challenge, which complicates the spraying and atomization process involved in SFD and GAS. For example, β-glucan (BG) with a MW up to 500 kDa can form solutions having viscosities ranging between 100 to 1,500 mPa·s at concentrations as low as about 1% (w/w) in water. Furthermore, the prior art is also silent when it comes to impregnation of such micro- or nanoparticles with bioactives or encapsulation of bioactives in micro- or nanoparticles made from such HMW biopolymers applying supercritical fluid technology for use in cosmetic, pharmaceutical, agricultural, nutraceutical or food products.